1. Field of the Invention
This invention relates to a printing method for reducing harmful effects on images such as color banding and gloss variations caused by what order to apply inks to a print medium in a serial type of color inkjet printing apparatus. In particular, the invention relates to a printing method of varying, depending on a print mode, a nozzle area used for printing in each of a plurality of nozzle arrays provided for each ink color to lessen the above-described harmful effects.
2. Description of the Related Art
A serial type of color inkjet printing apparatus prints an image by performing, in alternate order, a printing scan for ejecting ink toward a print medium while moving a plurality of nozzle arrays provided for each color and a conveying operation of conveying the print medium by a distance corresponding to a print width of the printing scan. Such an inkjet printing apparatus typically employs a multi-pass printing method for improving image quality or performs bidirectional printing for reducing the printing time. In the multi-pass printing, an image is completed in stages by performing a plurality of printing scans of the print head on an image region can be printed by one printing scan. Between the respective printing scans, the conveying operation is performed to convey the print medium by a distance shorter than the print width of the print head. By performing such multi-pass printing, dots are not printed continuously in the main scan direction by one single nozzle. This makes it possible to scatter variations in ejection characteristics among individual nozzles over the entire image, resulting in improved uniformity of the entire image
Now, it is known that, in color inkjet printing apparatus, the ink application order in which inks are applied to a print medium may possibly has an effect on color reproduction of an image. For example, when a single green image is printed by ejecting inks respectively from a cyan nozzle array and a yellow nozzle array which are arranged side by side in the main scan direction, if the image is printed in order of cyan and then yellow in the forward scan, the image is printed in order of yellow and then cyan in the backward scan. Such two images printed by reversing the ink application order differ from each other in color reproduction, so that color banding may possibly be recognized on a band-to-band basis.
It should be noted that, if the multi-pass printing as described above is performed, since ink is applied to a unit area of the print medium by both of printing in the forward scan and printing in the backward scan, the color banding can be lessened to some degree. However, in a case of a large amount of applied ink or a small number of passes in the multi-pass printing, it is difficult to obscure the color banding to perfection. If the number of passes is increased, the throughput is reduced.
In such circumstances, for example, Japanese Patent Laid-Open No. 2002-307672 discloses the structure in which, depending on a print mode, the operating area of the nozzle array in the conveying direction is changed independently for each color. According to Japanese Patent Laid-Open No. 2002-307672, for example, in a high-quality print mode of placing prime importance on image quality, the operating area in the nozzle array is set to a different position depending on an ink color such that the ink application order in which the inks are applied to a print medium is not reversed even during bidirectional printing. On the other hand, in a high-speed print mode of placing prime importance on printing speeds, the operating area of each nozzle array is extended to the maximum for printing.
According to Japanese Patent Laid-Open No. 2002-307672 as described above, it becomes possible to implement a high-quality mode for no occurrence of color banding and a high-speed print mode for a faster output speed in one single inkjet printing apparatus.
However, although Japanese Patent Laid-Open No. 2002-307672 discloses the structure of completely separating the operating areas of two nozzle arrays from each other in the conveying direction, that is, of preventing overlapping of the respective operating areas of two nozzle arrays in the conveying direction, it does not describe in detail the structure of using print heads for three or more colors. If such a structure as described in Japanese Patent Laid-Open No. 2002-307672 is implemented in a printing apparatus having nozzle arrays for three or more colors without any change, the number of nozzles actually used in the high-quality mode is extremely smaller than the number of nozzles of each nozzle array, resulting in a significant increase in print time.
Accordingly, a structure is employed actually, in which, while the operating areas of the nozzle arrays for two ink colors of which color banding are the most eye-catching are prevented from overlapping each other, a nozzle operating area for each of other ink colors is designed to partially overlap another nozzle operating area for another ink color of them. In such cases, however, according to the assiduous study of the inventors, it is found that color banding and gloss variations are produced depending on an amount of overlapping of a plurality of nozzle arrays overlapping with each other.